The present invention relates to a guide bar for a chain saw and, in particular, to methods of making such a guide bar.
Depicted in FIGS. 1 and 7 is a conventional guide bar (10) for a chain saw. The guide bar includes side rails (50) spaced apart by a middle rail (52) to form a groove (8) in which the drive links of a saw chain travel.
It is well known that the guide bars are subjected to excessive wear at their curved nose region where a saw chain exerts a great pressure on the curved edges even when the chain saw machine is idling without cutting wood, unless the saw chain is carried around the nose of the guide bar by a sprocket or roller. Guide bars with sprocket or roller noses have thus become the preferred design, unless the work environment is such that it is not possible to make a sprocket bearing with acceptable lifetime, such as when sawing has to be done in water or in other wet or abrasive environments. For such cases guide bars are often made with the nose faced with a smooth layer of some alloy having high wear resistance, usually cobalt-chrome-tungsten alloy, as described in U.S. Pat. No. 4,768,289 for example.
Several methods have been tried for the application of the alloy, including arc welding with or without an intermediate layer as described in U.S. Pat. Nos. 3,071,490 and 3,760,141, but these methods produce a thick rounded bead which afterwards has to be ground to the final shape, with a considerable waste of material. Another method suggested has been melting of alloy powder in a shaped mold held against the guide bar nose, as described in U.S. Pat. Nos. 5,144,867 and 5,448,929, but this method may produce porous alloy facing and also require grinding afterwards. It is also known to produce the alloy as a square section bar, which is bent around the guide bar nose and welded into place, as described in U.S. Pat. Nos. 3,241,228, 5,407,496 and 5,655,304, but this method requires a relatively soft alloy which may later be deformed in service even if it is not worn away. The alloy can also be made as precision cast pieces to be brazed onto the guide bar as disclosed in U.S. Pat. No. 3,858,321, but that is a slow and expensive method.
It is also known to laser weld a U-shaped strip (11) on the nose of each rail (50) as shown in FIGS. 1 and 7. It is conventional to form the strips from a plate (40') of a hard material arch as Stellite 6.RTM. (see FIG. 6). The plate has a thickness T (FIG. 6A) which corresponds to a width W of the rail (50) (see FIG. 7). The strips (11) are formed by a stamping process wherein strips (11A, 11B, etc.) are stamped successively from the plate. A crest (12) of the U-shaped strip has a width W1 which is greater than a width W2 at the ends (13) of the strip (see FIG. 6). Such a configuration enables waste of material to be minimized during the stamping-out procedure, and provides the greatest amount of wear-resistant material at the place of greatest wear on the guide bar nose.
However, it is necessary to prepare the plate (40') for the stamping process by cutting a wider plate (not shown) into sections (40') each having a width W3 corresponding to a desired length of each strip, which adds to the cost of forming the strips.
Also, it is conventional to surface harden the strips after they have been stamped out, but the resulting hardness is not uniform throughout the strip.
Thus, it would be desirable to provide a more economical method for forming the wear resistant strips.
It would also be desirable to provide wear resistant strips having a more uniform hardness.